Automotive exhaust system units

ABSTRACT

Automotive exhaust resonator or muffler having a tubular outer shell with a tapered or dome-shaped downstream end and exhaustconveying tube extending longitudinally therethrough and a hollow exhaust discharge extension having its upstream end snugly fitted on the medial portion of the shell, said extension embodying a downstream portion merging from the round or oval transverse cross section of the upstream portion into a substantially rectangular transverse cross section defining a rectangular exhaust discharge opening which is longer than the diameter of said medial portion in one dimension and narrower than said diameter in the other dimension.

United States Patent [72] Inventor Vincent E. Ignoffo 7040 W. Newport, Chicago, 111. 60634 [21] Appl. No. 93 [22] Filed Jan. 2, 1970 [45] Patented Feb. 9, 1971 Continuation-impart of application Ser. No. 744,917, July l5 1968, now abandoned.

[54] AUTOMOTIVE EXHAUST SYSTEM UNITS 12 Claims, 6 Drawing Figs. [52] US. Cl 181/59, 181/54, 181/63, 181/61, 181/72 [51] Int. Cl F01n l/04, F01n1/10,F01n 7/20 [50] Field ofSearch 181/48, 59, 54, 61-63, 72, 53 [56] References Cited UNITED STATES PATENTS 2,511,359 6/1950 McLeod l8 l/48X 3,348,629 10/1967 Cassel 181/48 3,404,750 10/1968 Powers et a1.... 181/48 3,500,954 3/1970 Willette 181/59X Primary ExaminerRobert S. Ward, Jr.

Att0rneyJohnson, Root, OKeefe, Keil, Thompson and Shurtleff ABSTRACT: Automotive exhaust resonator or muffler having a tubular outer shell with a tapered or dome-shaped downstream end and exhaust-conveying tube extending longitudinally therethrough and a hollow exhaust discharge extension having its upstream end snugly fitted on the medial portion of the shell, said extension embodying a downstream portion merging from the round or oval transverse cross section of the upstream portion into a substantially rectangular transverse cross section defining a rectangular exhaust discharge opening which is longer than the diameter of said medial portion in one dimension and narrower than said diameter in the other dimension.

AUTOMOTIVE EXHAUST SYSTEM UNITS This application is a continuation-in-part of my copendng application Ser. No. 744,9l7, filed July 15, I968.

INTRODUCTION The invention disclosed and claimed in the aforesaid copending application concerns improvements in straightthrough mufflers embodying a tubular shell with thicker wall, dome-shaped end portions. A tubular core extends axially through the shell. Sound absorbing means, such as woven or matted glass fibers, is provided in the annular space between the shell and the core. This annular space is connected with the tubular core by a plurality of louvers or vanes in the tubular wall of the core. The louvers extend transversely of the longitudinal axis of the core. They are arranged on the core in a plurality of circumferentially staggered columns or rows extending longitudinally along the core. The louvers are pitched relative to the longitudinal axis of the core with the openings between contiguous louvers facing the upstream end of the core. These louvers or vanes are arranged in the wall of the core preferably in at least four longitudinally disposed sets. Each set comprises a pair of said columns in substantially diametrically disposed relationship. The columns or rows of respective sets are circumferentially staggered in longitudinal sequence along the tubular core. The amount of circumferential staggering is in the range of about 45 to 90, preferably about 90.

The invention herein concerns hollow exhaust discharge extensions adapted to be fixedly mounted on the downstream end of such mufi'lers or on other mufflers or on resonators having a similarly shaped outer shell, i.e., a tubular outer shell having a medial portion of circular or oval, transverse cross section with a tapered or dome-shaped downstream end. The improvements of the subject invention are primarily related to those exhaust systems wherein the mufflers or resonators are located at the downstream end of the automotive exhaust system. The hollow exhaust discharge system fitted thereon and extending rearwardly therefrom thus becomes the visible portion of the exhaust system at the rear end of the automobile beneath the body.

The exhaust mufiler preferably is a muffler having the louvered tubular core of the type described in my aforesaid application. If desired, however, the tubular core may be provided with passages of other types, e.g., circular perforations, slits or the like in the tubular core.

The resonators are similar in some respects to the mufflers but generally do not embody a heat resistant fiber packing in the annular space between the outer'shell and the tubular core, such annular space being hollow or empty. One type of resonator utilizes a louvered tubular core of similar construction to the cores disclosed in my aforesaid application with the louvers located, however, only in the upstream portion of the core, e.g., the upstream half of the core. Another type of resonator utilizes an unperforated core with an annular opening at the upstream end of the resonator providing communication between the aforesaid hollow annular space and the upstream end of the tubular core. The primary purpose of such resonator is to remove low pitched sounds from the discharged exhaust. Such resonators as aforedescribed may be used in combination with straight-through mufflers of the type herein described or with mufflers having baffle systems within the hollow shell of the muffler to reduce exhaust noises.

THE INVENTION HEREIN portion of circular or oval, transverse cross section and a tapered or dome-shaped downstream end. An exhaust-conveying tubular core extends longitudinally through the shell between upstream and downstream openings in the respective ends of the shell. A hollow exhaust discharge system extension is mounted on the outer shell and serves as an exhaust-conveying discharge extension of the tubular core.

The exhaust discharge extension has an upstream portion of round or oval transverse cross section, the upstream edge of which is snugly fitted on the circular or oval, medial portion of the shell upstream from the dome-shaped or tapered end thereof. Such extension further has a downstream portion merging from the circular or oval, transverse cross section of its upstream portion into a substantially rectangular, transverse cross section. The latter cross section defines a rectangular exhaust discharge opening having a horizontal, longer dimension greater than the diameter of the upstream portion of the extension and a vertical, narrow dimension which is less than the diameter of said upstream portion.

The substantially rectangular downstream portion preferably has securely mounted thereon a hollow metal tip of substantially rectangular cross section conforming substantially to the rectangular cross section of the downstream part of the extension. This metal tip preferably is diagonally, transversly truncated at its downstream end in a manner providing a top wall of the metal tip which is wider than the bottom wall of the metal tip and diagonal downstream edges of the respective sidewalls.

In the preferred form of the hollow extension, it is formed from a cylindrical tube having a diameter larger than the diameter of the outer shell of the muffler or resonator. The upstream end of such cylindrical tube is swaged into a tapered or dome-shaped end portion until the diameter of the opening at the apex thereof is slightly less than the diameter of the outer shell. The downstream end of the cylindrical tube is formed into the rectangular shape simultaneously with the swaging operation or before or after such operation. The forming step may be achieved by pushing a tapered male die into the downstream end of the cylindrical tube to gradually reshape the initial circular cross section into a rectangular cross section. The resultant product is an open ended, hollow member having the first medial portion of larger diameter than the diameter of the outer shell, an upstream portion tapering from said medial portion to an upstream edge defining an upstream, coaxial opening in the frustoconical or frustohemispherical, tapering upstream portion, and a downstream portion defining a hollow section with an exhaust gas passage of substantially rectangular cross section, the dimensions of which rectangular cross section are a longer dimension which is greater than the diameter of said medial portion and a narrower dimension which is less than the diameter of said medial portion. A second medial portion of the resultant member is that portion which merges from the circular first-mentioned medial portion into the rectangular downstream portion.

This member is mounted on the tubular outer shell by placing its opening about the dome-shaped or tapered downstream end thereof and thereafter pressing said extension onto the shell to a position where the upstream edge of the extension passes beyond the tapered or dome-shaped end of the shell. In the process, the metal at the upstream opening of the hollow extension is deformed into a narrow, cylindrical neck which is so tightly fitted about the cylindrical, medial portion of the shell that it will not be disengaged therefrom; preferably, however, this neck is spot welded at three or four places to the outer shell as a precautionary measure against displacement of the extension from the shell during use on an automobile.

In an alternative, though less preferred form, the hollow extension is forrned from a cylindrical tube having an inner diameter the same as or only slightly larger than the outer diameter of the outer shell of the muffler or resonator. The upstream end of the extension is forced over the downstream end of the outer shell to a point where its upstream edge encircles the medial portion of the shell at a place upstream from the dome-shaped or tapered end. The downstream end of the extension is formed into the rectangular shape simultaneously with the aforesaid assembly operation or preceding or after said operation. The shaping of the downstream end into the rectangular cross section is achieved by the procedure aforedescribed. The resultant product is an open ended, hollow extension member having an upstream portion and first medial portion of substantially uniform circular or oval transverse cross section, a second medial portion merging from the first medial portion into a rectangular cross section, and a downstream portion defining a hollow section with a gas exhaust passage of substantially rectangular cross section. The dimensions of the latter passage are a longer dimension which is greater than the diameter of the upstream and first medial portions and a narrower dimension which is less than the diameter of said upstream and medial portions.

THE DRAWINGS Preferred embodiments of the invention are illustrated in the drawings wherein:

FIG. 1 is a diametric cross section of a glass-packed muffler with the preferred form of a hollow exhaust discharge extension mounted on the downstream end of the muffler;

FIG. 2 is a rear elevation of the embodiment of FIG. 1 as viewed from plane 2-2 of FIG. 1;

FIG. 3 is an exploded, perspective view of the downstream end of the outer shell of an exhaust sound-reducing member, e.g., the muffler of FIG. 1, and the two components of the holk w extension;

FIG. 4 is an exploded view in diametric cross section of the downstream portion of an automotive exhaust resonator and the two components of a less preferred form of the hollow exhaust discharge extension;

FIG. 5 is a diametric cross section of a louvered automotive exhaust resonator with the hollow exhaust extension embodiment as illustrated in FIGS. 1-3 mounted on the downstream end of the outer tubular shell of the resonator; and

FIG. 6 is a diametric section of another form of automotive exhaust resonator with the hollow extension of FIG. 4 mounted thereon.

THE PREFERRED EMBODIMENT The glass-packed muffler or resonator shown in FIG. 1 is described in detail in my pending application Ser. No. 744,917, the disclosure of which is incorporated herein by reference. This muffler is what is known in the muffler trade as a straight-through muffler 10. Its basic parts are a tubular shell 11, a straight-through core 12 and a fibrous, heat insulating, noise absorbent lining or packing l3 interposed therebetween. The tubular core 11 may be made of relatively light gauge sheet metal. Its upstream and downstream ends 14 and 15 have a dome shape which preferably provides dome walls 16 at opposite ends of the muffler having a thickness substantially greater than the thickness of the walls 17 of the major portion of the shell 11. The latter feature can be attained by swaging a cylindrically tubular shell having a uniform wall thickness as shown for the wall portion 17 in a swaging machine having a dome-shaped female die. As the end portions of the tubular shell are deformed inwardly by the swaging die, the metal in the dome-shaped ends 14 and 15 builds up to a thickness at least about L25 times the original wall thickness of the end portions of the shell. The extra thickness in the wall of the dome-shaped portions provides considerable improvements in the service life of the muffler.

The tubular core 12 extends axially through the tubular shell 11. It comprises a metal tube 20 having its upstream end 21 projecting through the axial opening 22 in the domeshaped end 14. The downstream end 24 of the core 12 preferably has a tubular expansion sleeve 25 tightly but slidably fitted thereover. The purpose of such sleeve is to relieve stresses in the tubular core 12 which would otherwise occur when the core 12 expands under the heat of exhaust gases flowing therethrough. The downstream end of the sleeve 25 may have a flared lip or shoulder 26 to preclude the projecting end of such sleeve from slipping into the axial opening 28 in the dome-shaped downstream end 15 of the outer shell I l.

openings 30 between vanes or louvers face the upstream endof the core. The openings 30 are defined along their upper edge by the arc of the upstream edge of one louver'or vane 31. corresponding substantially in curvature to thecircumfercncc of tubular core 12, and along the bottom edge by the substantially flat upstream edge of the immediately preceding vane or louver. Looking directly .into the openings 30, they have a shape substantially that of a portion of a circle cut by a chord. The depth of opening between the crest of the upper edge and the lower edge is generally about one sixteenth inch to one eighth inch.

The louvers or vanes 31 are arranged in sets composed of substantially diametrically opposite rows or columns 33-36. respectively. These sets are'respectivcly staggered on the circumference of the tubular core 12, most preferablyin an alternating sequence of about circumferential stagger as illustrated in the drawing. Each row or-column contains at least five vanes or louvers.

The vanes or louvers 31 may be formed in any suitable manner. One preferred technique is that of punching and forming the tubular wall of the core 12 into the vanes or louvers 31 in the arrangement'above described. As a general rule of thumb, there are about five to twelve louvers or vanes per column or row and four to eight setsof pairs of oppositely disposed columns or rows in the tubular cores 12. It is generally preferred that the number of vanes or louvers in each column or row be the same, but a variance of plus or minus two, or even .three in long tubular cores, vanes or louvers per column or row is tolerable. The circumferential stagger in longitudinal sequence of said rows or columns may be in the range of about 45-90 but most preferably, as aforesaid, is about 90.

The annular space between the core 30 and the shell 11 contains a heat-resistant, sound-absorbing, preferably fibrous material 40. Glass fibers are particularly well suited for this purpose because of their high heat resistance. The fibrous packing 40 may compose a sheet of loosely matted or loosely woven glass fibers wrapped about the core 12 in superposed layers 41 as shown in FIG. 1. Such packing can be conveniently installed in the muffler by wrapping it around the core and inserting the wrapped core into the shell 11 before both ends of the shell 11 are swaged into the domed shapes.

The hollow exhaust discharge extension 45 is made from a cylindrical or oval tube initially of substantiallygreater inner diameter than the outer diameter of the outer shell 11. It is formed by swaging the upstream end of the cylindrical or oval cross section tube into a tapered or dome-shaped upstream end portion 46 until the diameter of the opening 47 and the apex thereof is slightly less than the diameter or oval cross section of the outer shell 11. The downstream end of the cylindrical tube is formed into a hollow section 48 of substantially rectangular cross section. The resultant extension is an open ended, hollow member having a first medial portion 49 of larger diameter than the diameter of the outer shell. Its upstream portion tapers from the first medial portion to an upstream edge 50 defining the coaxial opening 47 in the frustoconical or frustohemispherical, tapering upstream portion 46. A second medial portion 51, downstream of the first medial portion 49, merges from the circular or oval cross section of the first medial portion into the rectangular downstream portion 48.

As can be seen in FIG. 2, the rectangular exhaust discharge opening 52 of the hollow extension 45 is longer than the diameter of the medial portion 49 in one dimension and is narrower than the diameter of said medial portion in the other dimension.

The hollow extension preferably has mounted on the rectangular, downstream end 48 a hollow metal tip 53. This tip has several functions. First, it serves as a decorative exposed end of the exhaust system, particularly when it is made of a bright metal such as stainless steel, chrome-plated steel, etc. Second, it reinforces the downstream end of the hollow extension. Third, by virtue of the relative dimensions and shapes of its walls, it aids in directing the discharge exhaust stream downwardly.

The metal tip 53 is a hollow rectangular member as viewed in transverse cross section. Its top wall 54 is wider than its bottom wall 55. The two sidewalls 56 and 57 are trapezoids having a diagonally directed downstream edge 58. The metal tip may be formed from a metal blank cut into the appropriate shape and bent to provide the aforesaid walls and transverse cross section. The bottom wall is composed of abutting wall sections 550 and 55b, the abutting ends of which may be welded together. The tip is fittedly secured on the rectangular downstream end 48 of the hollow extension 45 either by spot welding or by a seam weld 59. The hollow extension 45 may be similarly attached to the hollow shell 11 by spot welds or a seam weld 60. Three or four spot welds are sufficient in view of the fact that the upstream edge of the hollow extension 45 adjacent the apex opening 47 is deformed into a tight fitting cylindrical neck 61 which is sufficient in itself to secure the hollow extension tightly on the outer shell 11.

In the embodiment of FIG. 4, a resonator 65 has an outer shell 66 similar to the outer shell 11. An exhaust-conveying tube 67 extends longitudinally therethrough and is attached at the upstream and downstream ends to the shell 66 by welds 68. The annular space 69 between the tube 67 and the shell 66 is a hollow annular chamber devoid of packing, such as the glass fiber packing'l3 in the muffler of FIG. 1. The upstream end of the resonator of FIG. 4 has an exhaust-gas passage means communicating the tube 67 and the hollow annular space 69 (not shown in FIG. 4). Such exhaust gas-passage means may be those shown in FIG. 5 or FIG. 6, hereinafter described.

The hollow extension 70 is composed of an upstream segment 71 of circular or oval transverse cross section corresponding to the transverse cross section of the shell 66. The inner diameter of the segment 71, in the case where it is a hollow cylinder, is substantially the same as the outer diameter of the shell 66 whereby the segment 71 matingly fits over the downstream portion of the shell 66 as shown in FIG. 6. The extension 70 further has a medial portion 72 merging into the rectangular downstream segment 73. The latter has a shape and dimensions similar to the segment 48 in FIGS. l--3.

The metal tip in FIG. 4 is the same in structure and shape as the metal tip 53 in FIGS. 1-3, and like numerals have been used to designate like parts.

The resonator 74 of FIG. 5 and the hollow extension 45 mounted thereon are similar in structure and assembly to the straight-through muffler of FIGS. 1-3. Where applicable, like numerals designate like parts. The primary differences are (a) the hollow annular space 75 between the exhaust-conveying tube a and the shell 11 is devoid of packing, and (b) the exhaust-conveying tube 200 has louvers 3] only in the upstream portion, the downstream portion 76 of the tube being an unperforated wall. The louvers 31 are arranged in two sets of diametrically opposed longitudinal columns 33 and 34. The resonator 74 is used in an exhaust system containing upstream thereof either a conventional baffle muffler or a straightthrough mufiler of the type shown in FIG. 1. Its function is that of removing low pitched sounds from the exhaust gas discharge.

The resonator 80 of FIG. 6 is used in a similar manner in an exhaust system to the resonator of FIG. 5, i.e., in conjunction with a conventional bafi'le muffler or a straight-through muffler. It is composed of the outer shell 81 having a dome-shaped downstream end 82 and a tapered upstream end 83. A tubular adapter or connector 84 is attached by a bead weld or spot welds 85 in the upstream opening of the shell 81. An exhaustconveying tube 86 is rigidly attached to the downstream opening 87 by spot or head welds 88. The exhaust-conveying tube is unperforated. The exhaust gas passage means in the resonator of FIG. 6, which communicates the empty annular space 89 with the exhaust-conveying tube 86, is an annular passage 90 formed between the upstream ends of the exhaust-conveying tube 86 and the adapter or connector 84.

The hollow extension and the metal tip thereon correspond to that illustrated and described above with respect to FIG. 4. Accordingly, like numerals designate like parts.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the forms herein disclosed being preferred embodiments for the purpose of illustrating the invention.

I claim:

1. An automotive exhaust system unit comprising an exhaust sound-reducing member having a tubular outer shell with a medial portion of circular or oval, transverse cross section and a tapered or dome-shaped, downstream end having an opening in the apex thereof, an exhaust-conveying tubular core extending longitudinally through said shell into said opening, and a hollow, exhaust-discharge extension mounted on said outer shell in communication with the downstream end of said tube and serving as an exhaust-conveying extension of said tube, said discharge extension embodying an upstream portion of round or oval transverse cross section encircling the downstream portion of said shell with the upstream edge thereof snugly fitted on said shell upstream from said dome-shaped or tapered end and further embodying a medial portion merging its circular or oval, transverse cross section into a substantially rectangular, transverse cross section at the downstream, exhaust-discharge end thereof.

2. A unit as claimed in claim 1 wherein said substantially rectangular, transverse cross section defines a rectangular exhaust discharge opening longer than the diameter of said medial portion in one dimension and narrower than the diameter of said medial portion in the other dimension.

3. A unit as claimed in claim 2 wherein said downstream portion of substantially rectangular, transverse cross section has securely mounted thereon a hollow, metal tip of substantially rectangular, transverse cross section, a top wall wider than its bottom wall, and trapezoidal sidewalls with diagonal downstream edges.

4. A unit as claimed in claim I wherein said downstream portion of substantially rectangular, transverse cross section has securely mounted thereon a hollow, metal tip of substantially rectangular, transverse cross section, a top wall wider than its bottom wall, and trapezoidal sidewalls with diagonal downstream edges.

5. A unit as claimed in claim I wherein said exhaust discharge member has a first medial portion of greater diameter than the diameter of said shell, a frustohemispherical or tapered upstream portion with its upstream edge defining an upstream opening, said upstream edge adjacent said opening being deformed into a narrow, cylindrical neck tightly fitted about said shell, and a second medial portion merging from said first medial portion into said downstream portion of substantially rectangular, transverse cross section.

6. A unit as claimed in claim 5 wherein said substantially rectangular, transverse cross section defines a rectangular exhaust discharge opening longer than the diameter of said first medial portion in one dimension and narrower than the diameter of said first medial portion in the other dimension.

7. A unit as claimed in claim 6 wherein said unit is a glasspacked muffler, said exhaust-conveying tube having a plurality of perforations therein, and a packing of glass fibers in the annular space between said tube and said shell.

8. A unit as claimed in claim 5 wherein said unit is a glasspacked muffler, said exhaust-conveying tube having a plurality of perforations therein, and a packing of glass fibers in the annular space between said tube and said shell.

resonator having a hollow, annular space between said shell and said exhaust-conyeying tube. and exhaust gas-passage means communicating said tube and said hollow space.

12. A unit as claimed in claim I wherein said unit is a resonator having a hollow, annular space between said shell and said exhaust-conveying tube-and exhaust, gas-passage means communicating said tube and said hollow space. 

1. An automotive exhaust system unit comprising an exhaust sound-reducing member having a tubular outer shell with a medial portion of circular or oval, transverse cross section and a tapered or dome-shaped, downstream end having an opening in the apex thereof, an exhaust-conveying tubular core extending longitudinally through said shell into said opening, and a hollow, exhaust-discharge extension mounted on said outer shell in communication with the downstream end of said tube and serving as an exhaust-conveying extension of said tube, said discharge extension embodying an upstream portion of round or oval transverse cross section encircling the downstream portion of said shell with the upstream edge thereof snugly fitted on said shell upstream from said dome-shaped or tapered end and further embodying a medial portion merging its circular or oval, transverse cross sectioN into a substantially rectangular, transverse cross section at the downstream, exhaust-discharge end thereof.
 2. A unit as claimed in claim 1 wherein said substantially rectangular, transverse cross section defines a rectangular exhaust discharge opening longer than the diameter of said medial portion in one dimension and narrower than the diameter of said medial portion in the other dimension.
 3. A unit as claimed in claim 2 wherein said downstream portion of substantially rectangular, transverse cross section has securely mounted thereon a hollow, metal tip of substantially rectangular, transverse cross section, a top wall wider than its bottom wall, and trapezoidal sidewalls with diagonal downstream edges.
 4. A unit as claimed in claim 1 wherein said downstream portion of substantially rectangular, transverse cross section has securely mounted thereon a hollow, metal tip of substantially rectangular, transverse cross section, a top wall wider than its bottom wall, and trapezoidal sidewalls with diagonal downstream edges.
 5. A unit as claimed in claim 1 wherein said exhaust discharge member has a first medial portion of greater diameter than the diameter of said shell, a frustohemispherical or tapered upstream portion with its upstream edge defining an upstream opening, said upstream edge adjacent said opening being deformed into a narrow, cylindrical neck tightly fitted about said shell, and a second medial portion merging from said first medial portion into said downstream portion of substantially rectangular, transverse cross section.
 6. A unit as claimed in claim 5 wherein said substantially rectangular, transverse cross section defines a rectangular exhaust discharge opening longer than the diameter of said first medial portion in one dimension and narrower than the diameter of said first medial portion in the other dimension.
 7. A unit as claimed in claim 6 wherein said unit is a glass-packed muffler, said exhaust-conveying tube having a plurality of perforations therein, and a packing of glass fibers in the annular space between said tube and said shell.
 8. A unit as claimed in claim 5 wherein said unit is a glass-packed muffler, said exhaust-conveying tube having a plurality of perforations therein, and a packing of glass fibers in the annular space between said tube and said shell.
 9. A unit as claimed in claim 1 wherein said unit is a glass-packed muffler, said exhaust-conveying tube having a plurality of perforations therein, and a packing of glass fibers in the annular space between said tube and said shell.
 10. A unit as claimed in claim 6 wherein said unit is a resonator having a hollow, annular space between said shell and said exhaust-conveying tube, and exhaust gas-passage means communicating said tube and said hollow space.
 11. A unit as claimed in claim 5 wherein said unit is a resonator having a hollow, annular space between said shell and said exhaust-conveying tube, and exhaust gas-passage means communicating said tube and said hollow space.
 12. A unit as claimed in claim 1 wherein said unit is a resonator having a hollow, annular space between said shell and said exhaust-conveying tube, and exhaust gas-passage means communicating said tube and said hollow space. 